The present invention relates generally to grippers for downhole tractors to improved gripper assemblies.
Tractors for moving within underground boreholes are used for a variety of purposes, such as oil drilling, mining, laying communication lines, and many other purposes. In the petroleum industry, for example, a typical oil well comprises a vertical borehole that is drilled by a rotary drill bit attached to the end of a drill string. The drill string may be constructed of a series of connected links of drill pipe that extend between ground surface equipment and the aft end of the tractor. Alternatively, the drill string may comprise flexible tubing or xe2x80x9ccoiled tubingxe2x80x9d connected to the aft end of the tractor. A drilling fluid, such as drilling mud, is pumped from the ground surface equipment through an interior flow channel of the drill string and through the tractor to the drill bit. The drilling fluid is used to cool and lubricate the bit, and to remove debris and rock chips from the borehole, which are created by the drilling process. The drilling fluid returns to the surface, carrying the cuttings and debris, through the annular space between the outer surface of the drill pipe and the inner surface of the borehole.
Tractors for moving within downhole passages are often required to operate in harsh environments and limited space. For example, tractors used for oil drilling may encounter hydrostatic pressures as high as 16,000 psi and temperatures as high as 300xc2x0 F. Typical boreholes for oil drilling are 3.5-27.5 inches in diameter. Further, to permit turning, the tractor length should be limited. Also, tractors must often have the capability to generate and exert substantial force against a formation. For example, operations such as drilling require thrust forces as high as 30,000 pounds.
As a result of the harsh working environment, space constraints, and desired force generation requirements, downhole tractors are used only in very limited situations, such as within existing well bore casing. While a number of the inventors of this application have previously developed a significantly improved design for a downhole tractor, further improvements are desirable to achieve performance levels that would permit downhole tractors to achieve commercial success in other environments, such as open bore drilling.
In one known design, a tractor comprises an elongated body, a propulsion system for applying thrust to the body, and grippers for anchoring the tractor to the inner surface of a borehole or passage while such thrust is applied to the body. Each gripper has an actuated position in which the gripper substantially prevents relative movement between the gripper and the inner surface of the passage, and a retracted position in which the gripper permits substantially free relative movement between the gripper and the inner surface of the passage. Typically, each gripper is slidingly engaged with the tractor body so that the body can be thrust longitudinally while the gripper is actuated. The grippers preferably do not substantially impede xe2x80x9cflow-by,xe2x80x9d the flow of fluid returning from the drill bit up to the ground surface through the annulus between the tractor and the borehole surface.
Tractors may have at least two grippers that alternately actuate and reset to assist the motion of the tractor. In one cycle of operation, the body is thrust longitudinally along a first stroke length while a first gripper is actuated and a second gripper is retracted. During the first stroke length, the second gripper moves along the tractor body in a reset motion. Then, the second gripper is actuated and the first gripper is subsequently retracted. The body is thrust longitudinally along a second stroke length. During the second stroke length, the first gripper moves along the tractor body in a reset motion. The first gripper is then actuated and the second gripper subsequently retracted. The cycle then repeats. Alternatively, a tractor may be equipped with only a single gripper for specialized applications of well intervention, such as movement of sliding sleeves or perforation equipment.
Grippers are often designed to be powered by fluid, such as drilling mud in an open tractor system or hydraulic fluid in a closed tractor system. Typically, a gripper assembly has an actuation fluid chamber that receives pressurized fluid to cause the gripper to move to its actuated position. The gripper assembly may also have a retraction fluid chamber that receives pressurized fluid to cause the gripper to move to its retracted position. Alternatively, the gripper assembly may have a mechanical retraction element, such as a coil spring or leaf spring, which biases the gripper back to its retracted position when the pressurized fluid is discharged. Motor-operated or hydraulically controlled valves in the tractor body can control the delivery of fluid to the various chambers of the gripper assembly.
The prior art includes a variety of different types of grippers for tractors. One type of gripper comprises a plurality of frictional elements, such as metallic friction pads, blocks, or plates, which are disposed about the circumference of the tractor body. The frictional elements are forced radially outward against the inner surface of a borehole under the force of fluid pressure. However, these gripper designs are either too large to fit within the small dimensions of a borehole or have limited radial expansion capabilities. Also, the size of these grippers often cause a large pressure drop in the flow-by fluid, i.e., the fluid returning from the drill bit up through the annulus between the tractor and the borehole. The pressure drop makes it harder to force the returning fluid up to the surface. Also, the pressure drop may cause drill cuttings to drop out of the main fluid path and clog up the annulus.
Another type of gripper comprises a bladder that is inflated by fluid to bear against the borehole surface. While inflatable bladders provide good conformance to the possibly irregular dimensions of a borehole, they do not provide very good torsional resistance. In other words, bladders tend to permit a certain degree of undesirable twisting or rotation of the tractor body, which may confuse the tractor""s position sensors. Also, some bladder configurations may substantially impede the flow-by of fluid and drill cuttings returning up through the annulus to the surface.
Yet another type of gripper comprises a combination of bladders and flexible beams oriented generally parallel to the tractor body on the radial exterior of the bladders. The ends of the beams are maintained at a constant radial position near the surface of the tractor body, and may be permitted to slide longitudinally. Inflation of the bladders causes the beams to flex outwardly and contact the borehole wall. This design effectively separates the loads associated with radial expansion and torque. The bladders provide the loads for radial expansion and gripping onto the borehole wall, and the beams resist twisting or rotation of the tractor body. While this design represents a significant advancement over previous designs, the bladders provide limited radial expansion loads. As a result, the design is less effective in certain environments. Also, this design impedes to some extent the flow of fluid and drill cuttings upward through the annulus.
Yet another type of gripper comprises a pair of three-bar linkages separated by 180xc2x0 about the circumference of the tractor body. FIG. 21 shows such a design. Each linkage 200 comprises a first link 202, a second link 204, and a third link 206. The first link 202 has a first end 208 pivotally or hingedly secured at or near the surface of the tractor body 201, and a second end 210 pivotally secured to a first end 212 of the second link 204. The second link 204 has a second end 214 pivotally secured to a first end 216 of the third link 206. The third link 206 has a second end 218 pivotally secured at or near the surface of the tractor body 201. The first end 208 of the first link 202 and the second end 218 of the third link 206 are maintained at a constant radial position and are longitudinally slidable with respect to one another. The second link 204 is designed to bear against the inner surface of a borehole wall. Radial displacement of the second link 204 is caused by the application of longitudinally directed fluid pressure forces onto the first end 208 of the first link 202 and/or the second end 218 of the third link 206, to force such ends toward one another. As the ends 208 and 218 move toward one another, the second link 204 moves radially outward to bear against the borehole surface and anchor the tractor.
One major disadvantage of the three-bar linkage gripper design is that it is difficult to generate significant radial expansion loads against the inner surface of the borehole until the second link 204 has been radially displaced a substantial degree. As noted above, the radial load applied to the borehole is generated by applying longitudinally directed fluid pressure forces onto the first and third links. These fluid pressure forces cause the first end 208 of the first link 202 and the second end 218 of the third link 206 to move together until the second link 204 makes contact with the borehole. Then, the fluid pressure forces are transmitted through the first and third links to the second link and onto the borehole wall. However, the radial component of the transmitted forces is proportional to the sine of the angle xcex8 between the first or third link and the tractor body 201. In the retracted position of the gripper, all three of the links are oriented generally parallel to the tractor body 201, so that xcex8 is zero or very small. Thus, when the gripper is in or is near the retracted position, the gripper is incapable of transmitting any significant radial load to the borehole wall. In small diameter boreholes, in which the second link 204 is displaced only slightly before coming into contact with the borehole surface, the gripper provides a very limited radial load. Thus, in small diameter environments, the gripper cannot reliably anchor the tractor. As a result, this three-bar linkage gripper is not useful in small diameter boreholes or in small diameter sections of generally larger boreholes. If the three-bar linkage was modified so that the angle xcex8 is always large, the linkage would then be able to accommodate only very small variations in the diameter of the borehole.
Another disadvantage of the three-bar linkage gripper design is that it is not sufficiently resistant to torque in the tractor body. The links are connected by hinges or axles that permit a certain degree of twisting of the tractor body when the gripper is actuated. During drilling, the borehole formation exerts a reaction torque onto the tractor body, opposite to the direction of drill bit rotation. This torque is transmitted through the tractor body to an actuated gripper. However, since the gripper does not have sufficient torsional rigidity, it does not transmit all of the torque to the borehole. The three-bar linkage permits a certain degree of rotation. This leads to excessive twisting and untwisting of the tractor body, which can confuse the tractor""s position sensors and/or require repeated recalibration of the sensors. Yet another disadvantage of the multi-bar linkage gripper design is that it involves stress concentrations at the hinges or joints between the links. Such stress concentrations introduce a high probability of premature failure.
Some types of grippers have gripping elements that are actuated or retracted by causing different surfaces of the gripper assembly to slide against each other. Moving the gripper between its actuated and retracted positions involves substantial sliding friction between these sliding surfaces. The sliding friction is proportional to the normal forces between the sliding surfaces. A major disadvantage of these grippers is that the sliding friction can significantly impede their operation, especially if the normal forces between the sliding surfaces are large. The sliding friction may limit the extent of radial displacement of the gripping elements as well as the amount of radial gripping force that is applied to the inner surface of a borehole. Thus, it may be difficult to transmit larger loads to the passage, as may be required for certain operations, such as drilling. Another disadvantage of these grippers is that drilling fluid, drill cuttings, and other particles can get caught between and damage the sliding surfaces as they slide against one another. Also, such intermediate particles can add to the sliding friction and further impede actuation and retraction of the gripper.
In at least one embodiment of the present invention, there is provided an improved gripper assembly that overcomes the above-mentioned problems of the prior art.
In one aspect, there is provided a gripper assembly for anchoring a tool within a passage and for assisting movement of the tool within the passage. The gripper assembly is movable along an elongated shaft of the tool. The gripper assembly has an actuated position in which the gripper assembly substantially prevents movement between the gripper assembly and an inner surface of the passage, and a retracted position in which the gripper assembly permits substantially free relative movement between the gripper assembly and the inner surface of the passage. The gripper assembly comprises an elongated mandrel, a first toe support longitudinally fixed with respect to the mandrel, a second toe support longitudinally slidable with respect to the mandrel, a flexible elongated toe, a driver, and a driver interaction element. The mandrel surrounds and is configured to be longitudinally slidable with respect to the shaft of the tractor. The toe has a first end pivotally secured with respect to the first toe support and a second end pivotally secured with respect to the second toe support so that the first and second ends of the toe have an at least substantially constant radial position with respect to a longitudinal axis of the mandrel. The toe comprises a single beam.
The driver is longitudinally slidable with respect to the mandrel, and is slidable between a retraction position and an actuation position. The driver interaction element is positioned on a central region of the toe and is configured to interact with the driver. Longitudinal movement of the driver causes interaction between the driver and the driver interaction element substantially without sliding friction therebetween. The interaction between the driver and the driver interaction element varies the radial position of the central region of the toe. When the driver is in the retraction position, the central region of the toe is at a first radial distance from the longitudinal axis of the mandrel and the gripper assembly is in the retracted position. When the driver is in the actuation position, the central region of the toe is at a second radial distance from the longitudinal axis and the gripper assembly is in the actuated position. The second radial distance is greater than the first radial distance.
In another aspect, the present invention provides a gripper assembly for use with a tractor for moving within a passage. The gripper assembly is longitudinally slidable along an elongated shaft of the tractor. The gripper assembly has actuated and retracted positions as described above. The gripper assembly comprises an elongated mandrel, a first toe support longitudinally fixed with respect to the mandrel, a second toe support longitudinally slidable with respect to the mandrel, a flexible elongated toe, a ramp, and a roller. The mandrel is configured to be longitudinally slidable with respect to the shaft of the tractor. The toe has a first end pivotally secured with respect to the first toe support and a second end pivotally secured with respect to the second toe support. The ramp has an inclined surface that extends between an inner radial level and an outer radial level, the inner radial level being radially closer to the surface of the mandrel than the outer radial level. The ramp is longitudinally slidable with respect to the mandrel. The roller is rotatably secured to a center region of the toe and is configured to roll against the ramp. In a preferred embodiment, the toe preferably comprises a single beam.
Longitudinal movement of the ramp causes the roller to roll against the ramp between the inner and outer levels to vary the radial position of the center region of the toe between a radially inner position corresponding to the retracted position of the gripper assembly and a radially outer position corresponding to the actuated position of the gripper assembly. Preferably, the ramp is movable between first and second longitudinal positions relative to the mandrel. When the ramp is in the first position, the roller is at the inner radial level and the gripper assembly is in the retracted position. When the ramp is in the second position, the roller is at the outer radial level and the gripper assembly is in the actuated position.
In yet another aspect, the present invention provides a gripper assembly for use with a tractor for moving within a passage, the tractor having an elongated shaft. The gripper assembly has actuated and retracted positions as described above. The gripper assembly comprises an elongated mandrel, a first toe support longitudinally fixed with respect to the mandrel, a second beam support longitudinally slidable with respect to the mandrel, a flexible toe, a piston longitudinally slidable with respect to the mandrel, a ramp, a slider element, and a roller. The mandrel is configured to be longitudinally slidable with respect to the shaft of the tractor. The toe has a first end pivotally secured with respect to the first toe support and a second end pivotally secured with respect to the second toe support. The ramp is positioned on an inner surface of the toe. The ramp slopes from a first end to a second end, the second end being radially closer to the surface of the mandrel than the first end. The slider element is longitudinally slidable with respect to the mandrel and longitudinally fixed with respect to the piston. The roller is rotatably fixed with respect to the slider element and configured to roll against the ramp.
The ramp is oriented such that longitudinal movement of the slider element causes the roller to roll against the ramp to vary the radial position of the center region of the toe between a radially inner position corresponding to the retracted position of the gripper assembly and a radially outer position corresponding to the actuated position of the gripper assembly. The piston and the slider element are movable between first and second longitudinal positions relative to the mandrel. When the piston and the slider element are in the first position, the first end of the ramp bears against the roller and the gripper assembly is in the retracted position. When the piston and the slider element are in the second position, the second end of the ramp bears against the roller and the gripper assembly is in the actuated position.
In yet another aspect, the present invention provides a gripper assembly for use with a tractor for moving within a passage, the tractor having an elongated shaft. The gripper assembly has actuated and retracted positions as described above. The gripper assembly comprises an elongated mandrel, a first toe support longitudinally fixed with respect to the mandrel, a second toe support longitudinally slidable with respect to the mandrel, a flexible elongated toe, a slider element, and one or more elongated toggles. The mandrel is configured to be longitudinally slidable with respect to the shaft of the tractor. The toe has a first end pivotally secured with respect to the first toe support and a second end pivotally secured with respect to the second toe support. The slider element is longitudinally slidable with respect to the mandrel, and is slidable between first and second positions. The toggles have first ends rotatably maintained on the slider element and second ends rotatably maintained on a center region of the toe. The toe preferably comprises a single beam.
The toggles are adapted to rotate between a retracted position in which the second ends of the toggles and the center region of the toe are at a radially inner level that defines the retracted position of the gripper assembly, and an actuated position in which the second ends of the toggles and the center region of the toe are at a radially outer level that defines the actuated position of the gripper assembly. Longitudinal movement of the slider element causes longitudinal movement of the first ends of the toggles, to thereby rotate the toggles. When the slider element is in the first position the toggles are in the retracted position. When the slider element is in the second position the toggles are in the actuated position.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above and as further described below. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.